Structures that are to be held both level and above the ground may be set on continuous foundations such as poured concrete. Many structures are supported on columns which in turn are held on piers.
Piers are frequently in the form of concrete bases with large lower surfaces to engage the earth and upper surfaces on which columns of wood or steel may be set. A typical pier is a concrete truncated pyramid having a wooden plate set in it to which wooden columns such as four by fours are fixed. Variations in the grade of the earth above which a structure is supported can be accomodated by making the columns different lengths so that tops of all columns lie in the same horizontal plane.
When a group of piers and columns is used to support a structure there are problems in maintaining the tops of all columns in the same horizontal plane. In the first instance it is necessary to cut each column to be exactly the correct length even though each column may have a different length from each other column because the supporting piers are at different elevations. Cutting columns to be exactly the correct length is so difficult that it is substantially never accomplished correctly and small variations from the exact length required are adjusted by the use of shims between the top of a column and the supported structure.
Even after all columns and piers are properly in place and the structure is supported evenly above the earth, the earth on which the piers are set may yield because of the load applied, settling of back-fill, or for other reasons. Settling typically occurs beneath all structures and when some of the supporting elements settle and others do not, or when different supporting elements settle to different degrees it will cause some of the columns supporting the structure to carry more than their share of the load and in extreme cases other columns will carry no load at all. If some columns carry a small load or no load the beam span between columns becomes too long and eventually the beam sags causing undesirable effects of settling such as floors that are not horizontal, door and windows that do not function properly and cracks in ceilings and walls.
When a structure has been built and settling begins after a number of years the crawl space beneath the structure can be entered and those columns that have receded can be made useful again by employing shims between the column tops and the supporting structure. If further settling occurs the shimming process is repeated.
Self-adjusting supports have been used to avoid the problems mentioned above. Among the self-adusting supports that have been used are those in which the supporting columns are fixed to the structure they support and their bottoms are set in casings which surround the column bottoms. The casings include ground bearing elements and they surround the supporting columns and are filled or partially filled with flowable ganular material such as sand. When the earth settles beneath such a casing the casing may settle to a lower elevation but the column cannot because its top is connected to the supported structure. As the casing moves to a lower elevation sand between the casing and the column flows beneath the column thereby maintaining the casing in a supporting relationship with the column. Vibrations and flexing of the beams supported by the column cause the sand to flow to a position where firm support is maintained.
The problems found with devices such as those described above are that they must maintain a large space between the column and the casing to provide adequate inventory of sand and the sand inventory must be replenished periodically. The large space also permits the column to become nonvertical within the casing so that the load is supported by a column having a less than optimum orientation. In addition, some of the casings are exposed to the weather so that the sand inventory within them may become damp and less adapted to flow beneath the bearing surface of a column at or near the bottom of the casing.